1. Field of the Invention
The present invention relates to a radio terminal and a communication terminal which execute communications through a first network or a second network having a delay time shorter than a delay time in the first network.
2. Description of the Related Art
There has been conventionally known a radio terminal which executes real-time communications (for example, VoIP) with a communication terminal (corresponding node) through a network (see, for example, Japanese Patent Application Publication No. 2008-141266). In the radio terminal, a buffer which temporarily stores packets is provided in order to absorb a delay time and jitter in the network. The amount of packets to be stored in the buffer is determined according to the delay time in the network. Here, the concept of the delay time in the network includes not only a time period (stay time) in which the packet from the communication terminal (corresponding node) stays in the network but also variations of the stay time (jitter).
There has also been proposed an adaptive jitter buffer (AJB) control technique, targeting one network, for adjusting an amount of stored packets (jitter buffer size) by controlling a reproducing rate of the packets according to a delay time (stay time or variations of the stay time) of received packets. In the AJB control technique, when the amount of packets stored in the buffer exceeds an optimum packet amount, the packet reproducing rate is adjusted to a rate higher than a predetermined rate, so that the amount of stored packets is decreased. On the other hand, when the amount of packets stored in the buffer falls below the optimum packet amount, the packet reproducing rate is adjusted to a rate lower than the predetermined rate, so that the amount of stored packets is increased.
Meanwhile, there has been known a technique for executing handover from a first network to a second network. In the handover, a delay time in the second network may differ from a delay time in the first network. In such a case, the handover causes a change in the optimum packet amount as the amount of packets to be stored in the buffer.
For example, suppose the case where the delay time in the first network is longer than the delay time in the second network. In such a case, an appropriate packet amount in the second network is smaller than an appropriate packet amount in the first network. Accordingly, after the handover from the first network to the second network is executed, the amount of packets stored in the buffer becomes unnecessarily large. As a result, the real-time constraint of the packets is lost.
As described above, the AJB control technique is implemented for one network. Accordingly, in the operation using the AJB control technique, the radio terminal sharply decreases the amount of packets stored in the buffer when the handover from the first network to the second network is executed. In other words, the radio terminal sharply increases the packet reproducing rate.
As seen from above, it is difficult to properly control the amount of packets stored in the buffer after the handover from the first network to the second network is executed.